Monitoring of chemicals in drinking water as well as in sewage water faces several major difficulties. First, a wide variety of chemicals should be monitored. For example, Nitride may be monitored since Nitride chemical competes with hemoglobin over oxygen and a high concentration of Nitride in water can cause so-called blue baby syndrome by preventing hemoglobin to arrive to blood. Techniques to monitor Nitride generally involve electrodes which become polluted thereby degrading quickly their efficiency. Ammonium may also be monitored since it is toxic to fish and other aquatic organisms. Phosphate may also be monitored since phosphate may cause development of alga and water weed which consume oxygen and destroy the water resources. Phosphate monitoring is especially important in agriculture and in sewage purification facilities. Techniques to monitor phosphate generally involve laboratory analysis and do not provide online real time analysis i.e. the results cannot be provided on site and have to be taken to a laboratory. Chloride may also be monitored since Chloride enable to determine the penetration of salty water in fresh water. Techniques to monitor Chloride generally also involve laboratory analysis. An alternative indirect measurement of Chloride may also be based on measuring conductivity although it is not precise and reliable. Boron may also be monitored in order to prevent damages to animals and plants. Particularly, real time Boron measurements in water desalination processes are of especially high importance since the removal of Boron during desalination processes is performed at high pH and pressure and is therefore costly. Thus, the ability to accurately measure the level of Boron may enable to avoid undue treatment.
In fact, techniques generally provide for continuous monitoring of only very few parameters like rate of flow, pressure, turbidity, pH, electrical conductivity and concentration of free Chlorine and do not provide with continuous monitoring of other chemical components concentrations. Even though some other techniques enable to detect a large plurality of chemical components, these alternative techniques adversely require laboratory analysis and do not provide with real time results. Further, most of the previously discussed techniques can only detect simultaneously a single chemical and generally require complicated maintenance operations which may include additions of chemical reagents, constant calibrations and high costs.
Spectrometry systems (also generally referred to as spectrometers) are used to measure properties of light over a specific portion of the electromagnetic spectrum. Spectrometry systems may advantageously be used to identify materials comprised in a sample according to the properties of light retrieved from the sample in so-called spectroscopic analysis. For example, Raman spectroscopy provides with a real time alternative to the aforementioned techniques. As described in L. A. Lyon, C. D. Keating, A. P. Fox, B. E. Baker, L. He, S. R. Nicewarner, S. P. Mulvaney and M. J. Natan, “Raman Spectroscopy,” Anal. Chem. 70, 341R-361R (1998) Raman-effect-based techniques are capable of providing identification of materials as well as estimating concentration of said materials in a sample. The Raman effect (also referred to as Raman scattering) is an optical non linear effect that is used in spectroscopy as a tool for mapping and detecting materials. The technique is based on illuminating the sample with a monochromatic incident radiation and on measuring a wavelength shift between the incident radiation and the reflected or transmitted radiation. The shift characterizes the material and allows its identification. The amount of absorption i.e. the value of the shifted peak may assist in estimation of the concentration of said material. The aforementioned technique produces valuable estimations and has been considered for designing sensors in chemical and biological contamination events. FIG. 1 illustrates the basic principle of Raman scattering in case of a Stokes shift. Raman scattering is an effect in which an illumination photon 1 of the incident radiation induces inelastic scattering in the non linear regime of a material 2. The scattering generates a Raman photon 3 with lower frequency (and energy) while the energetic difference (and therefore the frequency difference) is passed to the vibration states 4 of the material 2. Usually there are two shifts of wavelength in the reflected or transmitted radiation. In this inelastic interaction of the photons, the phonons are either created (Stokes shift) or annihilated (anti-Stokes). In one case the wavelength is increased (called anti-Stokes) and in the other it is decreased (called Stokes). Since the anti-Stokes radiations have generally a lower intensity than the Stokes radiations, only Stokes radiations may be considered in Raman spectrometry. The difference between the wavelengths of the Raman radiation and the incident radiation is only dependent on the properties of the material and not on the properties of the incident radiation. Therefore, it is possible to determine a Raman signature of the material. More particularly, the Raman signature of the material may be obtained as a Raman shift i.e. a difference between the Raman radiation wavelength and the incident radiation wavelength. Therefore, it is possible to detect the presence of the material in a sample by detecting a peak corresponding to the Raman radiation in the frequency spectrum of the reflected radiation. The position of the Raman radiation in the frequency spectrum may be detected around the wavelength of the incident radiation shifted of the Raman shift corresponding to the material. Further, knowing the wavelength of the incident radiation, the frequency spectrum to analyze may be reduced to a spectral band around the expected position.
A Raman spectroscopy configuration for detecting a predetermined material and estimating the concentration of said material in a sample has been therefore proposed in the prior art and involves a laser arranged at one side of a monitoring cell receiving the sample and a spectrometer at the other side of the monitoring cell. The spectrometer detects the presence of said material based on the presence of the Raman signature of the material in the frequency spectrum provided by the spectrometer. Indeed, the existence of the predetermined material in the monitoring cell generates an expected Raman shift. Further, the intensity of the Raman radiation i.e. the value of the shifted peak is proportional to the concentration of the material in the sample. Therefore, both types of information—presence and concentration—can be extracted.
General Description
However, spectrometry systems face difficulties in determining the presence of certain materials due to spectral resolution limitation of the spectrometer. For example, Raman spectrometry systems do not enable determining the presence in water of chemicals such as Boron, Arsenic, Perchlorate salts and heavy metals especially when said chemicals are present at low concentrations. Indeed, the spectral signatures of certain materials may be highly problematic to detect in a sample.
The present invention provides with a system and method which provides online and real time detection of a plurality of materials contained in a sample even when the spectral signatures of said materials are spectrally close and/or even when the concentration of said materials is very low. The sample may be a liquid, gas, solid, gel, slurry, powder, films, etc. In an embodiment, the sample may be water. In order to do so, the present invention proposes to super resolve the wavelength spectral information in a spectrometry configuration.
Resolution of an imaging system is defined as the capability to distinguish between two adjacent spatial features and consider them as two rather than a single larger one. The field of super resolution is the field in which the spatial information is encoded in such a way that another domains such as time, coherence, field of view, polarization or gray level are used in order to convert the spatial degrees of freedom that could not be resolved by the imaging system, and later on to decode them while constructing the spatially super resolved image. The process of conversion of spatial degrees of freedom into other domains is called multiplexing. For example, using the time domain to super resolve is called time multiplexing. In order to do this conversion of degrees of freedom one needs to know that the domains, into which the conversion takes place, have the availability to perceive the spatial information. For instance, if one uses time multiplexing and converts the spatial information into the temporal domain, he needs to know that the object being imaged is not varying in time during the time slot needed in order to increase the missing spatial resolution.
The added value in the spectral super resolving is related to obtaining an improved capability of detecting various chemicals (materials) as well as in the estimation of their concentration. This may be usefully applied to real time monitoring and evaluation of the quality of water. Therefore, a simplified, low cost multi functional monitoring system and method are presented. The present invention enables providing on-line and real time simultaneous estimation of several important materials in water as well as in other molecular samples such as gas, gels, solids, slurries, powders, films etc. The operation principle is based upon a new super resolved concept for spectrometry systems, particularly for Raman spectrometry. Although, as mentioned above, the temporal multiplexing is applied to super resolve the spectral distribution coming from a spectrometer. The operation principles may be understood in light of spatial information super resolving methods adapted to the spectrometry systems. By applying the proposed super resolving technique the obtainable spectral resolution can be significantly enhanced and leads to improved accuracy in the real time and in the on-line estimation capability of existing chemicals and their concentration in water as well as in other molecular samples such as gas, gels, solids, slurries, powders, films etc. The higher spectral resolution may also lead to the capability of monitoring several chemicals simultaneously with much higher accuracy in respect to the estimation of their concentration. For example, usage of super resolved spectral capability in Raman spectroscopy may provide detection capability with improved accuracy and precision of various chemicals in water e.g. Nitride, Ammonium, Phosphate, Chloride, Boron, Arsenic, Perchlorate salts, heavy metals, etc. Since the sharpness as well as the shape of the Raman peak is a finger print designating a specific material, the spectral resolution depends on its concentration. The height of the Raman peak is linked to the concentration of the chemical. The relation between spectral resolution and concentration is more or less linear. Therefore improvement of one order of magnitude in spectral resolution, as can be performed in the proposed super resolving approach, may yield one order of magnitude improvement in minimal detectable level of concentration and in its accuracy.
Following the description presented hereinafter it can be seen that a spectral resolution improvement can be obtained whenever there is a relative movement between the spectrum of the encoding medium and the spectrum that one wishes to super resolve. When using Raman spectroscopy a way to obtain relative spectral movement may be to use a fixed spectral encoding element and to tune the spectrum that we wish to map by changing the excitation wavelength of the Raman laser. However, in other types of spectroscopy one may realize the proposed concept as well by shifting the encoding spectrum function. This can be obtained in various ways for instance if the spectral encoding is realized via “handmade” devices such as predesigned spectral filters or various modified transmissions based upon Fabry-Perot resonators, then the spectral tuning can be obtained by applying electrical field, heating or mechanical movement over those filters in order to modify their spectral transmission (e.g. if one changes the distance between the mirrors of a Fabry-Perot resonator, its spectral transmission peaks will move). Other type of spectral encoding can be obtained by using “natural” means, e.g. to have a gas or a liquid or a solid having a defined transmission spectrum which fits our encoding purposes. In this case changing the pressure, volume or temperature of a gas (also for liquids and solids but in smaller amount) can modify its “natural” transmission curve.
An interesting point in the proposed technique is that most optical medium may be used as long as we know the spectral transmission curve of the medium in advance and in high resolution. Then the proposed processing can be performed enabling super-resolution. Still, the spectral transmission curve of the optical medium should contain high resolution spectral features (e.g. many sharp spectral peaks) in order to enable spectral encoding of the inspected spectral distribution.
Therefore, in a first aspect the presently disclosed subject matter provides a system for improving the resolution of a spectrometer, the spectrometer being configured to provide a frequency spectrum of a radiation incoming from a sample. The system comprises an optical medium configured so that the radiation incoming from the sample be transferred through the optical medium, the optical medium having a predetermined tunable spectral transmission curve; an operating unit connectable to the optical medium and configured to operate the optical medium so as to shift the spectral transmission curve of the optical medium over a predetermined spectral range; and a processing unit connectable to the spectrometer and configured to process a set of shifted frequency spectra provided by the spectrometer and obtainable by transferring the radiation incoming from the sample through the optical medium while shifting the spectral transmission curve of the optical medium so as to obtain a super resolved frequency spectrum of improved spectral resolution.
In a variant of the first aspect, the presently disclosed subject matter provides a system for improving the resolution of a spectrometer, the spectrometer being configured to provide a frequency spectrum of a radiation incoming from a sample illuminated with a tunable source of coherent radiation for generating a Raman emission in the sample. The system comprises: an optical medium configured so that the Raman emission generated by the sample is transferred through the optical medium, the optical medium having a predetermined spectral transmission curve; an operating unit connectable to the tunable source of coherent radiation and configured to shift a wavelength of the tunable source over a predetermined spectral range; a processing unit connectable to the spectrometer and configured to process a set of shifted frequency spectra provided by the spectrometer and obtainable by transferring the radiation incoming from the sample through the optical medium while shifting the wavelength of the tunable source so as to obtain a super resolved frequency spectrum of improved spectral resolution.
In some embodiments of the first and second variants, the optical medium is a tunable spectral filter.
In some embodiments of the first and second variants, the optical medium is a spectral filter sensitive to electrical field change.
In some embodiments of the first variant, the operating unit is configured to modify an electrical field of the optical medium so as to shift the spectral transmission curve of the optical medium over a predetermined spectral range.
In some embodiments of the first and second variants, the optical medium is a spectral filter sensitive to temperature, pressure and/or volume change.
In some embodiments of the first variant, the operating unit is configured to change a temperature, pressure and/or volume of the optical medium so as to shift the spectral transmission curve of the optical medium over a predetermined spectral range.
In some embodiments of the first and second variants, the optical medium is based on a Fabry-Perot resonator.
In some embodiments of the first variant, the operating unit is configured to modify a distance between two mirrors of the Fabry-Perot resonator so as to shift the spectral transmission curve of the optical medium over a predetermined spectral range.
In some embodiments of the first and second variants, the spectral transmission curve of the optical medium comprises spectral features of a spectral width smaller than a spectral resolution of the spectrometer.
In some embodiments of the first and second variants, the spectrometer is configured to detect a predetermined material in the sample and the spectral features are present in the spectral transmission curve of the optical medium in a spectral band comprising the wavelength of the predetermined material spectral signature.
In some embodiments of the first and second variants, the spectral band in which the spectral features are present has a spectral width substantially equal to the spectral resolution of the spectrometer.
In some embodiments of the second variant, the operating unit is configured to linearly shift the wavelength of the coherent source of radiation.
In some embodiments of the first variant, the operating unit is configured to linearly shift the spectral transmission curve of the optical medium.
In some embodiments of the first and second variants, the predetermined spectral range is at least as wide as a spectral resolution of the spectrometer.
In some embodiments of the first and second variants, the processing unit is configured to perform opposite time shifting, digital multiplication by the predetermined spectral transmission curve of the optical medium and time average on the set of shifted frequency spectra.
In some embodiments of the first and second variants, the optical medium is a gas phase.
In some embodiments of the first and second variants, the optical medium is a Bragg filter.
In some embodiments of the first and second variants, the processing unit is configured to detect in the sample any combination of chemicals, bacteria, medicine and drug.
In a second aspect, the presently disclosed subject matter provides a spectrometry system comprising a spectrometer configured to provide a frequency spectrum of a radiation incoming from a sample and a system according to any embodiment of the first or second variant of the first aspect previously described.
In some embodiments, the spectrometry system further comprises a tunable source of coherent light for generating a Raman emission in the sample.
In a third aspect the presently disclosed subject matter provides a water quality monitoring device comprising a monitoring cell or a basin configured to receive a water sample intended to be analyzed; and a system according to any embodiments of the first and second aspects previously described.
In a fourth aspect, the presently disclosed subject matter provides in a first variant a method of improving the resolution of a spectrometer configured to provide a frequency spectrum of a radiation incoming from a sample. The method comprises transferring the radiation incoming from the sample through an optical medium, the optical medium having a predetermined tunable spectral transmission curve; providing the radiation output by the optical medium to a spectrometer; shifting the spectral transmission curve of the optical medium over a predetermined spectral range; acquiring a set of shifted frequency spectra corresponding to a set of shift values of the shift of the spectral transmission curve; and processing the set of shifted frequency spectra so as to obtain a super resolved frequency spectrum of better resolution than the frequency spectrum.
In a second variant of the fourth aspect, the presently disclosed subject matter provides a method of improving the resolution of a spectrometer configured to provide a frequency spectrum of a radiation incoming from a sample. The method comprises illuminating a sample with a coherent radiation for generating a Raman emission in the sample; transmitting the Raman emission generated by the sample through an optical medium, the optical medium having a predetermined spectral transmission curve; providing the Raman emission output by the optical medium to a spectrometer; shifting a wavelength of the coherent radiation over a set of shifted wavelengths within a predetermined spectral range; acquiring a set of shifted frequency spectra corresponding to the Raman emissions transmitted through the optical medium for the set of shifted wavelengths; and processing the set of shifted frequency spectra so as to obtain a super resolved frequency spectrum of better resolution than the frequency spectrum.
In some embodiments of the first and second variants of the fourth aspect, the method further comprises determining if a predetermined material is contained in the sample based on the super resolved frequency spectrum.
In some embodiments of the first and second variants of the fourth aspect, the method further comprises determining a concentration of the material in the sample based on the super resolved frequency spectrum.
In some embodiments of the first and second variants of the fourth aspect, the spectral transmission curve of the optical medium comprises spectral features of a spectral width smaller than a spectral resolution of the spectrometer.
In some embodiments of the second variant of the fourth aspect, the spectral features are present around the wavelength of the Raman emission of the predetermined material to be detected.
In some embodiments of the second variant of the fourth aspect, shifting a wavelength of the coherent radiation comprises linearly shifting the wavelength of the radiation.
In some embodiments of the first and second variants of the fourth aspect, the predetermined spectral range is at least as wide as a spectral resolution of the frequency spectrum.
In some embodiments of the first and second variants of the fourth aspect, processing the set of shifted frequency spectra comprises opposite time shifting, digital multiplication by the predetermined spectral transmission curve of the optical medium and time average on the set of shifted frequency spectra.
In some embodiments of the first and second variants of the fourth aspect, the method further comprises detecting in the sample a concentration of any of nitride, ammonium, phosphate, chloride and boron based on the super resolved frequency spectrum.
In some embodiments of the first and second variants of the fourth aspect, the method further comprises detecting in the sample a concentration of any combination of chemicals, bacteria, hormone, medicine, drug, etc.
In some embodiments of the first and second variants of the fourth aspect, detecting in the sample comprises simultaneously detecting the concentration of any combination of chemicals, bacteria, hormone, medicine, drug, etc.